riscv: fix build warning of mm/pageattr

Add hearder for missing prototype. Also, static keyword should be at
beginning of declaration.

Signed-off-by: Zong Li <zong.li@sifive.com>
Reviewed-by: Pekka Enberg <penberg@kernel.org>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>